1. Field of the Invention
The present invention relates to a power transmission apparatus provided which acts as a torque limiter during an over-load condition and is suitably applied to a power transmission apparatus for operating a compressor located in a refrigerating system for an air conditioning system of an automobile.
2. Background of the Invention
In a refrigerating system for an air conditioning system of an automobile, it common that a compressor of a variable displacement type is used, wherein the capacity of the compressor can be varied even to a value which is nearly zero. In such an air conditioning system, the compressor can be continuously operated while the capacity is varied, i.e., an ON-OFF operation of the compressor is unnecessary, which allows elimination of a clutch device, which must otherwise be arranged between the crankshaft of an internal combustion engine and the compressor.
On the other hand, in the operation of the compressor, lockup can be generated due to overheating, which causes a belt to be subjected to slippage in the transmission system of the compressor, which may cause the belt to be damaged. Furthermore, it is usual for the belt for operating the compressor to be used for operating various supplementary devices of the internal combustion engine such as a battery charger/generator, a recirculating pump for an engine cooling water, and a oil pump for a power steering device and the like. Thus, damage to the belt may possibly cause the automobile to be immobilized.
Thus, provision of a torque limiter, wherein a power transmission train from a drive source to a compressor is disconnected when a transmission torque is increased to a value larger than a predetermined value during an over-load condition of a clutch as caused by an occurrence of a locking, has heretofore been proposed. See, for example, Japanese Unexamined Patent Publications No. 8-159028 and No. 8-210250.
In these prior patents, a torque limiter is constructed by a pulley rotated by a driving force of the engine, a base plate connected to the pulley, a driving force receiving member connected to a rotating shaft of the compressor arranged to face the base plate at a desired spacing, torque transmission balls which are rotatably received in recesses formed in the base plate and the driving force receiving member, and a spring means for generating a spring force between the base plate and the driving force receiving member so that the spacing therebetween is reduced.
In a normal operation, a condition is maintained that the rolling balls are held between the recess in the base plate and the driving force receiving members, while a power transmission from the connecting plate connected to the pulley to the driving force receiving member connected to the drive shaft of the compressor occurs by way of the rolling balls, which allows the compressor to be subjected to the compression operation.
An overload in the compressor due to locking causes the transmission torque to be increased to a value larger than a predetermined value, resulting in the rolling balls being detached from recesses in the base plate and the driving force receiving member against the spring force of the resilient means. The rolling ball is, for example, detached from the recess in the connecting plate on the side of the pulley. As a result, the rolling ball is disengaged from the connecting plate on the side of the pulley, which allows the connecting plate to be freely rotated with respect to the ball, thereby causing the compressor to be disconnected from the rotating movement from the source.
In the above mentioned prior art, a structure is employed for obtaining a switching between an engaged condition where the rolling ball is stored in the recess and a released condition where the rolling ball is disengaged from the recess. In other words, in this prior art structure, the rolling ball is an essential element. However, the rolling ball is formed as a spherical shape of a small diameter, which makes its handling difficult, thereby inevitably increasing the number of steps for completing the assembly of the compressor, resulting in an increase in a production cost.